X-ray crystal structure research aimed at determining the molecular structures of biologically important compounds will be carried out. Special emphasis will be on the development of improved methods for interpreting the x-ray diffraction patterns of complex materials in terms of the structure of the molecules. Methods applicable to molecules possessing non-crystallographic symmetry will be used in the detailed structure analysis of a crystalline plant virus, and possibly to other viruses. Several crystalline forms of cholesterol will be studied with methods applicable to molecules of partly known structure, present in crystals of unusual complexity. Work on the further development of a recently completed program for macromolecular least-squares refinement is also planned.